This invention relates to mechanical positioning systems, and, more particularly, to a linkage system for controllably and precisely positioning an article at discrete locations defined by the linkage.
It is often necessary to mechanically position an article at one of several precisely defined, discrete locations, and then to move the article between those locations. A variety of complex devices are available to perform such functions. A high-precision torque motor operating in conjunction with a positional encoder is one example of such a positioning device. Such devices can be extremely costly to purchase and may have limited reliability due to their complexity.
In an example of a requirement for precisely positioning an article at discrete locations, a sensor platform in an earth surveillance satellite is rotatably mounted so that its sensor may be aimed at locations along its ground track. In one version, the sensor platform must be capable of being precisely directed in one of three nominal discrete angular orientations: straight down (0 degrees reference), +20 degrees forward, or -20 degrees backward along the ground track. There is no requirement that the sensor platform be arbitrarily aimable at angles other than these nominal positions.
In one approach to meeting this requirement, a high-precision, microprocessor-controlled torque motor with an optical or electrical positional encoder feedback may be used to achieve precise positioning. The torque motor is connected to the rotatable sensor platform. The encoder measures the actual position of the sensor platform. In operation, the proper encoder position for a required angular orientation selected and commanded by the microprocessor. The encoder-based feedback system confirms that the command has resulted in the required angular orientation of the sensor platform or, if not, provides information for making the required adjustment of the torque motor.
Such a torque motor/encoder system can cost on the order of $250,000. It is complex, and therefore more prone to breakdowns and limited service life than an otherwise comparable but less complex device. The system also has the shortcoming that it provides more capability than required in some aspects, and less in others. For example, the torque motor/encoder system permits precise positioning at angular orientations other than the required three discrete positions. On the other hand, because the encoder is essentially a digital device, its increments of angular positioning at the discrete orientations is limited by the size of the digital steps and therefore may have difficulty achieving the required three discrete positions with a high degree of accuracy. Other types of precision positioning units are available, but generally also have high costs, are complex, and suffer from various shortcomings.
There is a need for an improved method and apparatus for precisely positioning an article to a discrete angular or linear location. The apparatus should be relatively inexpensive, provide long life and high reliability, and produce good controllability to the nominal location or orientation. The present invention fulfills this need, and further provides related advantages.